Patent GB-638661-A (GB)
Title: Improvements in radio signalling systems
IPC: G01S1/02
Filed: 1946-05-02
Abstract: 638,661. Radio navigation; multiplex radio signalling. SANDEMAN, E. K. May 2, 1946, No. 13278. [Classes 40 (v) and 40 (vii)] In a time division amplitude-modulated pulse multiplex system in which separate channels are communicated to each of a plurality of azimuthal sectors, the time allocation is performed at the receiving point under the control of a spiral phase field, i.e. a field in which the radio-frequency or modulation-frequency phase is a function of the azimuthal position with respect to the transmitter. In the beacon system of Fig. 1, a modulation frequency fs from oscillator 01 is fed through phase-splitter PS and amplifiers A1, A2 to produce four phases in quadrature relationship which are fed to the modulators M1-4 which are supplied with carrier frequency from 02 via phasing amplifiers PA1-4. The four modulated carriers from transmitters T1-4 are radiated from separate aerials N, E, S and W, each of which has a semi-cos 6 polar diagram, Fig. 3 (not shown), with the result that the field at any point bears a modulation at frequency fs, the phase of the modulation being the same as the azimuthal position of that point with respect to some datum direction; if required, the phase of the modulation could be made an integral multiple of the bearing. The frequency fs is also fed through a phasing amplifier PA5 to the control system DC of a distributer comprising gates G1-8 which is effective to connect sequentially to a modulator M5, eight message sources MS1-8 corresponding to the separate sectors with which separate communication is to be obtained. The eight sequential messages which may comprise bearing identifying morse characters, speech or other intelligence from any suitable source, recorded or otherwise, are modulated upon a carrier frequency from 03, differing from that producing the spiral phase field, the modulated carrier from transmitter T5 being radiated omnidirectionally; alternatively, the spiral phase modulation and the intelligence may be radiated on different subcarriers modulated on to a common carrier. At a receiving point, Fig. 2, the modulation frequency fs is extracted from the spiral phase field in receiver R and after passage through phase-adjusting amplifier PA is squared at SQ and differentiated at DF to produce at the output of amplifier A3 a rectangular pulse which is fed to open the gate GR in the receiver comprising signal frequency circuits SF, mixer Mx and local oscillator 04, and i.f. amplifier I.F. which receives the omnidirectional intelligencebearing transmission. The phase and duration of the gate pulse are so arranged that the i.f. signal passed to the detector D corresponds only to the message intended for the sector in which the receiver is located. The detected pulse after lengthening at PL to occupy the whole time interval until the next pulse is received, i.e. the duration of one period of fs and the rotation of the spiral phase field, is fed to any suitable circuit to utilize the amplitude modulation appearing on the pulse train. The pulse lengthening circuit PL may be of the type described in Specification 582,758. If the message is in the form of telegraph code and is required to be presented in the form of a modulated tone, the pulse waveform may be fed to a modulator M supplied from an audio oscillator 05 as shown.